It is known that during a tension spring forming operation, a twist may be introduced in the spring wire, such that adjacent coils tend to press against one another and resist subsequent deflection of the spring. A spring of this type is considered to have an "initial tension" or a "pre-load" (Pi), which must be overcome by a tension force applied to the spring before the spring coils will open or separate. The amount of initial tension present in a tension spring, which is formed of a given spring material, wire diameter "d" and mean spring diameter "D", may be controlled within limits depending upon the degree of bending of the wire incidental to the coiling of such wire to form the spring, and the degree to which the formed spring is subsequently stressed relieved by a heat treating operation.
Initial tension or preload is an important factor in obtaining a desired spring load at a desired spring length and in changing spring loads for a given spring design without changing wire size, spring diameter, the number of coils and the slope of the spring gradient or spring constant. As by way of example, the full extension for a given tension spring, i.e., that is the extension required to he applied to extend a spring to its highest load without producing permanent deformation of the spring, may be increased or decreased by changing the initial preload placed in the spring as it is formed. It is also oftentimes desirable to design a tension spring having the lowest spring gradient possible over a certain range of extension, and one way this has been accomplished is to initially wind the spring with maximum pretension. This produces a spring in which substantial spring force is available for use before extension of the spring is initiated and thus the spring may be formed with more coils, while still being fitted within a given space.
An empirical formula developed by Hunter Spring of Hatfield, Pa., a division of Ametek, Inc., from results from springs of various materials and proportions indicates that the greatest amount of initial tension, which can be obtained in a conventionally formed close-wound tension spring, after normal stress relieving, is ##EQU1## wherein "Sy" is the maximum apparent elastic limit for common spring materials appearing in a publication entitled Spring Design Data, copyright 1964 Ametek, Inc. The maximum apparent elastic limit appears to substantially exceed the actual "elastic limit" of such spring materials, due to certain beneficial internal stresses produced in the wire, during forming of the spring. The disclosure of this publication is incorporated by reference herein.